a. Field of the Invention
This invention relates to solid state lasers. More specifically, the invention is directed to an improved solid state laser including a plurality of solid state lasing elements, each having low thermal focusing and low thermal birefringence loss, which improved laser produces increased TEM.sub.00 mode power.
b. Background of the Pertinent Art
High power TEM.sub.00 solid state laser output, either continuous wave (CW) or Q-switched, is required for many commercial, scientific research and development applications. For example, diamond inscribing, laser engraving, soldering and welding applications and thick film hybrid circuit trimming operations require high power laser output. Numerous other industrial and scientific applications would also benefit if higher power laser output could be economically and practically employed.
A large mode volume in the solid state lasing crystal, i.e., utilizing as large a fraction of the laser crystal's active volume, is a critical requirement in achieving high power, low order mode laser output. Unfortunately, the mode volume in neodymium (Nd) doped yttrium aluminum garnet (YAG), commonly denoted as Nd:YAG, the most popular solid state lasing crystal, has been limited by the properties of that material known as thermally-induced focusing and thermal birefringence loss.
Thermally-induced focusing or thermal lensing results from the quadratic variation in the refractive index of the Nd:YAG rod as a function of distance from its central axis. This variation follows, via the large positive dn/dT, from the quadratically varying temperature dependence which results when a uniformly pumped (and heated) cylindrical rod is cooled on its cylindrical surface. Since the effective focal length of a highly pumped rod can be as short as 20-30 cm (i.e., approaching the rod length), the establishment of large mode volumes and the use of multiple rods within a single resonator is made difficult.
In addition, the radial temperature distribution results, via the thermal expansion coefficient, in a varying stress distribution. This stress produces, through the photoelastic effect, a birefringence in the normally optically isotropic Nd:YAG rod. This thermal birefringence causes a depolarization and hence an intracavity loss for a TEM.sub.00 mode beam as this mode is necessarily linearly polarized. Since this loss varies quadratically as a function of radial position from the rod center, the thermal birefringence also limits mode volume and thereby TEM.sub.00 mode power.
Therefore, CW TEM.sub.00 power over 30 watts remains unavailable from any commercial solid state laser system today.
Certain characteristics of the crystal Nd:LiYF.sub.4 (referred to herein as Nd:YLF) have been shown in recent studies to make Nd:YLF advantageous for use as an active lasing element. This crystal has been shown to demonstrate very low thermal focusing and, because it is a uniaxial crystal, its natural birefringence dominates the relatively small thermal birefringence so that thermal birefringence loss is negligible. In addition, Nd:YLF lasers have achieved TEM.sub.00 power outputs comparable to Nd:YAG lasers with equivalent sized rods.
Thermal focusing in Nd:YLF laser rods, however, although small, is astigmatic, i.e., its magnitude and sign are different in the orthogonal radial directions of the crystal a and axes in a typical laser rod whose other a axis is along the cylinder axis. This can be corrected by utilizing a cylindrical lens to produce an output beam that is circularly symmetrical. Such a cylindrical lens is used in addition to a spherical lens normally used to expand beam diameter so as to obtain larger mode volumes.